Subject: How does a Robin know where the worm is?

Upon observing a Robin in my back yard recently, I noticed it runs around 
the lawn, stops, cocks it's head like it's listening to something, pauses, 
then suddenly stabs at the ground with it's beak.70% of the time, it comes 
up with a worm or grub. Is the bird really listening for the sound of it's 
prey or what. How does it know where to stab? Is it sensing ground 
vibrations, it it actually seeing something in the grass? It's a mystery 
to me. 

Re: How does a Robin know where the worm is?